1. Field of the Invention
This invention relates generally to a time code generator and is directed more particularly to a time code generator which includes an error correcting circuit.
2. Description of the Prior Art
When a video signal is recorded by using a VTR (Video Tape Recorder), it is general that a time code signal for indicating an absolute time of the video signal is simultaneously recorded together with the video signal.
As the time code thus described, an SMPTE (Society of Motion Picture and Television Engineers) time code and an EBU (European Broadcasting Union) time code are respectively normalized for NTSC (National Television Systems Committee) and CCIR (International Radio Consultative Committee). This time code signal is formed of a binary signal for showing an hour, a minute, a second and a frame number thereof in every frame of the video signal, which is used, for example, as an address signal upon electronic edition of a tape.
It is normal that in many cases, the time code to be recorded employs a real time when the VTR is in the recording mode, but, in this case, such a drawback is brought about that the time code is made discontinuous at the place the recording is interrupted.
On the other hand, a time code generator having a function what is called as JAM SYNC has been proposed and according to this time code generator, it is possible that after the recording is interrupted the time code is recorded by a kind of extrapolations as the time code being successive to the preceding time code. However, in a conventional time code generator, if a dropout will occur in the time code signal, its successiveness is broken and becomes unstable, or the circuitry thereof becomes complicated and hence the time code generator is expensive.
To notice the fact that the tape of the VTR is transported at the normal speed when the time code generator is operated in the JAM SYNC mode, the inventors of this invention succeeded in making a time code generator which is simple in construction and which can operate reliably or positively.
FIG. 1A diagrammatically shows a pattern of a time code signal portion recorded on the tape of the VTR in which a letter E represents an end of the recording. According to the JAM SYNC mode, if the recording is made after the end E, a time code such as 9, 10 . . . is recorded following "8" as shown in FIG. 1A. Therefore, it becomes necessary to provide the time code generator to produce such signal. To realize this, the tape is switched to the reproducing mode after the tape is rewound by a little amount beyond the aforesaid end or position E, and the content of the time code read out from the tape is made coincident with the output of the time code generator and thereafter, a time code which can increment at every frame is generated, so that a time code with a code pattern as seen in FIG. 1B can be produced. Accordingly, if the magnetic head for producing the time code is switched to the recording mode at the end or editing point E so as to record the time code thus produced, the successive time codes can be obtained (arrows seen in FIGS. 1A and 1B diagrammatically represent directions along which the signals are supplied).
But, in the practical operation, there may occur such a possibility that any dropout is caused in the reproduced time code and the reproduced time code is read out erroneously. Thus, it becomes necessary to cope with the above aspect. As the error correction method, such method is proposed by the same assignee of the present invention in which is disclosed U.S. Pat. No. 4,360,841. But, when the above method is used for the JAM SYNC, there is a defect that the circuitry is complicated and hence many parts are needed therefor.
Accordingly, noticing the fact that the tape of the VTR is transported at the normal speed in JAM SYNC mode and the time code read out therefrom must always be the same as the time code produced from the time code generator except a case when any dropout is caused, the inventors of the present invention developed a time code generator which is simple in construction and which can operate positively or reliably.